Optical shutters use an actuator to drive each of one or more radiation-blocking elements or “shutter blades”, between a first, closed position that blocks the path of light through at least a portion of an aperture and a second, open position that is spaced apart from the first position and that allows light through the aperture. The light radiation that is directed toward the aperture can generally be any form of electromagnetic radiation, such as ultra-violet, visible or infrared radiation, for example. The aperture can be in a frame that is directly or indirectly coupled to the actuator. The frame can additionally support the actuator and typically includes features that retain the shutter blade or blades and that define the travel path of the shutter blade or blades. The actuator can be electromagnetically activated (an “electromagnetic actuator”) so that it responds to an electrical signal to translate the shutter blade or blades between the open and closed positions. Electromagnetic actuators typically used for this purpose include linear solenoids, rotary solenoids, or brushed or brushless commutated motors, for example.
Actuators for optical shutters can be monostable or bistable. Monostable shutters have a single stable position to which the actuator returns when power is removed. Bistable actuators are able to remain in the last position held at the time power is removed.
Monostable solenoid actuators have a coil of wire that generates a magnetic field when electrical power is applied. The magnetic field applies a force to pull or rotate a soft magnetic core in a given direction. Monostable actuators with soft magnetic cores typically utilize a spring or other mechanical element to return the core to an original position when power is removed. One disadvantage of monostable actuators for shutter control relates to their behavior upon power loss; these actuators require continuous power to remain in the electrically driven state.
Bistable actuators are stable in the state held when power is removed, whether open or closed. Bistable actuators can be created using geared motor drives that lock in a given position when unpowered. In other embodiments, an over-center spring can be used to create a locking force in either of the open aperture or closed (blocked aperture) positions.
The soft magnetic core of a monostable solenoid can be replaced with a hard magnet that adheres to soft magnetic material in each of its two positions to create a bistable shutter. For example, the rotary drive solenoids (RDS) produced by Melles Griot are exemplary bistable rotary solenoids, each using a permanent magnet core. Further description of bistable actuators of this type can be found, for example, in Proceedings of SPIE, Vol. 6542, “Advanced electro-mechanical micro-shutters for thermal infrared night vision imaging and applications” by Durfee et al. Bistable actuators are advantaged for their small size and light weight. However, these actuators have their limitations. Because they typically have relatively small coil elements, bistable rotary actuators used for shutter applications can be damaged by the application of continuous power and are typically pulsed intermittently so that energy can be more quickly dissipated. These devices can be constrained in terms of travel arc, allowing the blade to swing over an arc of 20 degrees or less between open and closed positions. This, in turn, tends to limit the size of the aperture.
Thus, it can be seen that there is a need for an improved shutter apparatus that uses a bistable actuator that operates in a monostable mode, returning the shutter blade to a preferred position when power is removed, and that allows a larger aperture to be provided.